This invention relates generally to high frequency modulated interferometry and is particularly directed to a Stark-tuned, modulated far-infrared (FIR) laser particularly adapted for measuring rapidly changing plasma electron densities in a magnetically confined plasma.
A number of approaches are currently being evaluated in the development of nuclear fusion as a long term energy source. Research in this area has lead to the finding that one of the most efficient configurations for optimum plasma containment is in the form of a toroid or donut. This has given rise to the tokamak fusion reactor design which is currently under intensive study by research groups in a number of countries. In this approach, a circular arrangement of powerful magnets forms a toroidal magnetic field wherein is confined an energetic plasma comprised primarily of protons and deuterons. Confining the highly energetic plasma at extremely high temperatures and densities causes the fusing of atoms, such as deuterium and tritium, and the resulting production of energy. High energy electrons arising from charge exchange collisions are also produced in the plasma.
Measurement of plasma electron density has proven to be an important diagnostic tool in measuring various characteristics of the magnetically confined, energetic plasma. Plasma electron density is typically performed using a frequency modulated interferometer employing far-infrared (FIR) lasers. In this approach, a laser beam is modulated by either mechanical means, such as by a rotating grating, or by optical means involving the beating together of two frequency-shifted FIR lasers. In the former case, modulation frequencies are limited by mechanical constraints to 104-10.sup.5 Hz, while in the latter case modulation is limited to about 1 MHz because of the very narrow gain bandwidth exhibited by submillimeter lasers. In practice, because of the relatively low modulation frequency of current interferometer systems, rapid changes in the density of the confined plasma result in a loss of fringe count in the interferometric measurements. Rapid plasma density changes are caused by the injection of solid pellets into the plasma core, by sudden loss of confinement due to major plasma disruption, or by fast magnetohydrodynamic (MHD) activity excited in the plasma core. Each of these phenomena has been the cause of density changes which are rapid enough to cause fringe loss in the plasma diagnostic interferometer system such as that employed in the tokamak fusion test reactor (TFTR) presently operating at Princeton Plasma Physics Laboratory. Prior approaches for measuring plasma electron density in the TFTR have made use of a pair of CH.sub.3 OH lasers operating at 119 microns and at a modulation frequency of 1 MHz. Not only does the limited bandwidth of prior art laser systems limit plasma density measurement accuracies, but the limited output power of these systems also restricts their utility as plasma diagnostic tools.
The present invention is intended to overcome the aforementioned limitations of the prior art by providing a high frequency, modulated laser system for use in an interferometer system for accurately measuring electron density in a high temperature, magnetically confined plasma. The laser system employs a Stark-tuned laser operating on the 119 micron line of CH.sub.3 OH having an increased output power while exhibiting a higher frequency doublet splitting modulation frequency. The laser includes a resonator cavity design which affords the aforementioned high output power and high frequency doublet splitting in a system particularly adapted for use with a multi-channel interferometer such as employed on large-scale plasma devices used in magnetic fusion research.